Many assay devices are now available commercially which are intended for quick simple tests in professional circumstances (clinics and doctor's offices) or in the home (for example pregnancy tests). Typically such assays are based on specific binding reactions, and therefore use sensitive reagents such as antibodies. A typical test device will comprise a strip of porous material within a plastics casing. The reagents are contained within the device in the dry state, and when sample liquid (such as urine) is applied to the device it migrates through the porous material and initiates one or more specific binding reactions which lead to the test result. Examples of such devices are described in EP-A-291194 the disclosures of which are included herein by reference.
These assay devices are manufactured and distributed on a large scale, and need to have a lengthy shelf life. The user expects the assay to be highly sensitive and therefore any deterioration in the performance of the assay reagents during storage would be unacceptable. To this end it is common practice for such devices to be provided in individual moisture-impervious packaging such as an aluminium foil pouch. A desiccant is provided within the pouch together with the assay device. Conventionally this is in the form of a small sachet of silica gel. As an alternative, a compressed tablet of desiccant (usually molecular sieve material) can be incorporated within the device itself. Both of these approaches have significant disadvantages.
The necessity to include a sachet of desiccant inside each foil pouch complicates the packaging process. More important, the sachet of desiccant is of no value to the consumer and must be discarded with the rest of the packaging. It can easily become separated from the remainder of the packaging. If mislaid in the home it may be found by an inquisitive child and perhaps ingested with potentially disastrous consequences.
The inclusion of a tablet of desiccant within the device itself adds a further step to the assembly process, and necessitates moulding features to accommodate and retain the tablet. In terms of storage stability there does not appear to be any extra benefit associated with having an internal tablet of desiccant compared to the effectiveness of an external sachet.